1. Field of the Invention
The present invention relates to a rewritable optical information recording medium including two substrates assembled together and a method for producing the same.
2. Description of the Related Art
Technologies for performing high-density recording of information and reproduction of such information using laser beams are known. For example, optical disks have been put into practical use. Optical disks are roughly classified into read-only type, write-once type, and rewritable type. The read-only type is realized as products such as, for example, compact disks having musical information stored therein and laser disks having image information stored therein. The write-once type is realized as products such as, for example, document files and still picture files. Currently, research and development is mainly performed on rewritable type. The rewritable type is being realized as products such as, for example, data files for personal computers.
A general optical disk has a transparent resin substrate having a thickness of 1.2 mm, a recording layer provided on one surface of the substrate, and a protective layer such as, for example, an overcoat provided on the recording layer. Another general optical disk includes a substrate and a protective plate formed of the same material as the substrate which are assembled together with an adhesive.
In order to increase the recording density of optical disks, studies have been recently performed for shortening the wavelength of the laser light and using an objective lens having a large numerical aperture (NA). However, as the wavelength of the laser light is shortened and the numerical aperture is increased, the tolerance for the angle of the disk with respect to the incident angle of the laser light (referred to as the "tilt") is reduced. Reducing the substrate thickness (distance from the surface of the substrate to the recording layer) is effective in increasing the tolerance for the tilt. For example, the substrate thickness of a digital video disk (DVD) is 0.6 mm. Since the resin substrate having a thickness of 0.6 mm is insufficient in mechanical strength, two such substrates are assembled with the recording layer interposed therebetween.
The substrates are assembled by various methods, such as, for example, applying a hot melt resin on a surface of one substrate and then putting the substrates into contact with each other, assembling the two substrates with an adhesive tape (two-sided tape) interposed therebetween, or applying a ultraviolet (UV) curable resin on a surface of one substrate, then putting the two substrates into contact with each other and curing the resin with ultraviolet rays.
It has been found that a substrate having a thickness of as small as 0.6 mm is significantly warped when provided with a thin layer including a rewritable recording layer on a surface of the substrate.
Such a phenomenon does not occur when a substrate having a thickness of 0.6 mm is provided with a thin layer including a read-only recording layer, i.e., a metal reflective layer (formed of, for example, Al or Au) on a surface of the substrate. The phenomenon does not occur with a substrate having a thickness of 1.2 mm regardless of a thin film, including a rewritable recording layer or a read-only recording layer is provided.
The cause of the warp is considered to be as follows. In the case of the rewritable type disks, a dielectric layer is included in the thin layer in addition to the recording layer for protecting the recording layer. Formation of the dielectric layer generates a large stress, and the stress warps the substrate which has an insufficient mechanical strength.
When such a significantly warped substrate and another substrate which is not warped are assembled together to product an optical disk capable of one-side recording and reproduction, the resultant optical disk is also warped. Such a warped optical disk is not usable.
When two substrates which have the same thin layers and thus are warped at the same degree are assembled, a highly flat optical disk is obtained because the stresses generated in both substrates are balanced. However, use of two substrates both having the recording layers for producing an optical disk for one-side recording disadvantageously raises the production costs.